Chemesthesis

Vol. 2 No.1 November 1999

Chemo S ense SPECIAL AACSS ISSUE

What is it that makes an orange tangy, a glass of beer refreshing and gives Chemesthesis: Chicken Tandoori its bite? It’s pain! In this issue Wayne Silver reviews the The burning questions trigeminal or chemesthetic sense, revealing how the irritation of a special By Wayne Silver, Wake Forest University bundle of nerves plays a major role in the flavour and aroma of foods. Animals have evolved a variety of chemoreceptive organs to sense chemicals in Also in this issue, Bets their external environment. Gustation and olfaction (taste and smell), are usually Rasmussen investigates the biggest recognised as the ‘chemical senses’. However, there is another sense that we use to nose of all — that of the majestic detect chemical stimuli. This sense was named the ‘common chemical’ sense by G.H. elephant. Elephant olfaction is leading to Parker in 1912 to describe the sensory system responsible for detecting chemical an understanding of how smell works at irritants. a molecular level. Rasmussen has Although Parker noted that the common chemical sense was mediated by free shown how male elephant mucus combines with female pheromones as a nerve endings resembling pain receptors, he concluded that “the common chemical bridging step to reproductive signalling. sense is a true sense with an independent set of receptors and a sensation quality The female elephant’s urine is lifted by entirely its own”. the tip of the male’s trunk and directed Today we know that the free nerve endings do not constitute a separate, independ- into the ducts of a part of the nose called ent sense. Rather, they are part of the general somatic sensory system: a subset of the vomeronasal organ (VNO). The pain- and temperature-sensitive fibres which can be found throughout the skin and elephant VNO tells the male that the mucosal membranes of the nose, mouth, respiratory tract, eye, and anal and genital female is about to ovulate. Humans have orifices. a VNO visible only through a microscope, whereas the elephant’s is the size of your The chemesthetic sense finger. Does size matter? And if humans These nerve endings also respond to chemicals, including irritants, and the term have similar pheromones how do they ‘chemesthesis’ has recently been used to describe the sensations elicited by their reach the human nose? And what messages do they carry? chemical stimulation. We stimulate our chemesthetic sense every time we eat chilli As a special feature this peppers, sniff ammonia, or cut onions (see Box, next page). There are many important 16 page issue includes abstracts from reasons for seeking to understand our chemesthetic sense, not the least of which is to the most recent meeting of the Australian better appreciate its role in flavours and aromas. Association for ChemoSensory Science. In humans, nerve fibres in the trigeminal (Vth cranial) nerve, innervating the mouth, From the world of business, nose and eyes, best exemplify chemesthesis Kemal Avunduk, an Australian living and (see Figure 1). While free nerve endings working in Japan, brings us some advice originating from some other cranial nerves also INSIDE: on judicious use of the tongue. respond to chemical stimuli, much of the Chemesthesis Plus, ChemoSense brings you research on chemesthesis, especially as it the latest news and upcoming events in relates to taste and smell, involves trigeminal the chemosensory world. chemoreception. Business in Japan Some nerve fibres in the trigeminal nerve respond to touch, some to cold, some to heat, and some to ‘painful’ stimuli. Those fibres that Elephant Olfaction respond primarily to chemical irritants are called capsaicin-sensitive polymodal nociceptors. AACSS Abstracts cont. pg 2 Centre for ChemoSensory Research In the News The University of New South Wales ISSN 1442-9098 1 CHEMESTHESIS continued

That is, they are receptors that respond to a variety of noxious, irritating stimuli (mechanical, thermal, and chemical) and are especially sensitive to capsaicin (the active, ‘hot’ ingredient in chilli peppers). When capsaicin stimulates these nociceptors it elicits a burning pain sensation. Recently, a capsaicin receptor (VR1) has been isolated. The receptor is a non-selective cation channel in the nerve membrane, activated not only by capsaicin, but also by high temperatures in the noxious range (Caterina et al., 1997).

That burning sensation A variety of sensations are elicited by stimulation of trigeminal chemoreceptors, including those described as pungent, tingling, stinging, burning, cooling, warming, painful, and irritating. In addition to providing a sensory experience, chemical stimulation of trigeminal chemoreceptors activates protective reflexes, such as increased secretion, decreased breathing, sweating, and decreased nasal patency. Anyone who has bitten into a chilli pepper or taken a whiff of ammonia has experienced some of these reactions. A curious aspect of chemesthesis is that, although many Figure 1 Branches of the chemically sensitive trigeminal nerve chemesthetic substances are initially (and probably innately) in the regions of the mouth and nose. aversive, they eventually become preferred. Examples include tobacco, chilli pepper, mustard, curry, horseradish, ginger, and vinegar. Infants, children, and uninitiated adults typically reject these substances on first exposure. Most adult humans, however, reverse their natural aversion and develop strong Figure 1 positive responses to at least one initially unpalatable sub- ‘Pungency’: stance. Indeed, whole cuisines, such as Mexican and Thai, The Trigeminal Chemosense (Chemesthesis) are based on some of these chemesthetic stimuli. How the reversal of the initial aversion occurs is not well understood but The sensations arising from the chemesthetic sense are any explanation has to account for the inability to produce described as ‘hot’, ‘warm’, ‘pain’, ‘burn’, ‘bite’, ‘sting’, ‘prickle’, preferences for these compounds in most other animals. ‘itch’, ‘tingle’, ‘fizz’, ‘numbness’, ‘freshness’, ‘chill’, ‘coolness’ and ‘coldness’. Putting the tingle, fizz and bite into foods Foods that invoke the chemesthetic sense include: A number of important questions remain to be answered chilli, pepper, mustard, ginger, onion, garlic, horseradish, concerning chemesthesis. We do not yet know to what extent peppermint, cumin, coriander, cinnamon, cloves, spearmint, it contributes to our perception of odour, tastes, or flavours. carbonated drinks, sherbet, salt, vinegar, fruit powders and food Could chemesthetic stimulation enhance or diminish percep- acids. tion? Research is also currently under way to determine Chemical compounds that act on the Trigeminal nerve whether different chemesthetic stimuli elicit different sensa- endings: tions. For example, can people lacking olfactory systems discriminate between different irritants based on the quality of capsaicin (from chilli) piperine (from pepper) gingerol (from ginger) zingerone (from ginger) those irritants? If the answer to this is yes, it suggests that allyl isothiocyanate (from mustard) different stimuli have specific receptors. This could be an cinnamaldehyde (from cinnamon) important finding that would allow the creation of compounds cuminaldehyde (from cumin) specifically targeted to bind to specific receptors and elicit 2-propenyl/2-phenylethyl isothiocyanate (from horseradish) specific sensations. Perhaps some day we may be able to carbon dioxide (bubbles in carbonated liquid) purchase ‘designer’ hot sauces and seasonings to spice up some salts, including sodium chloride our foods in new ways. some acids, including acetic and citric acid All of this is good news for those of us who like food that alcohol , ammonia, nicotine, menthol bites back. thymol (from thyme) eucalyptol (from eucalyptus plants) Caterina, M.J., Schumacher, M.A., Tominaga, M., Rosen, T.A., Levine, J.D., Julius, D. (1997). The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389:816-824. 2